1. Field of the Invention
The present invention relates to a method of crystallizing an amorphous silicon layer on a substrate. More particularly, the present invention relates to a method of crystallizing an amorphous silicon layer without damaging the glass substrate by a high temperature process.
2. Discussion of the Related Art
Thin film transistors manufactured using amorphous silicon are generally fabricated at lower temperatures than transistors using single-crystal or polycrystalline silicon, but they present a disadvantage in that their charge carriers have a low mobility in channel regions. Thin film transistors using polysilicon typically have non-uniform characteristics, because the ability of their charge carriers depends on a degree of crystallization, and in general there is fluctuation in such degree of crystallization over a substrate. This is generally not true for thin film transistors using single-crystal silicon. Polycrystalline thin film transistors are also more affected by the grain boundary effects, where the carriers in channels have to pass through crystal grain boundaries, because polysilicon contains more crystalline grains than single-crystal silicon. In a thin film transistor having a channel region that is formed by crystallizing amorphous silicon, each grain of crystal should be formed as large as possible to reduce the number of crystalline grains and enhance the carrier mobility. The size of crystalline grains formed through amorphous silicon crystallization by a laser beam depends on the energy density of the laser beam, temperature of the substrate, and the rate of the crystallization. It is important, therefore, to properly adjust these parameters to regulate the production and growth of crystals from amorphous silicon.
Amorphous silicon is crystallized by melting using a laser, and allowing the silicon to solidify and cool. This produces crystalline grains that act as seed crystals that gradually grow into large crystalline grain. Each of crystalline grains has an arbitrary but fixed crystal orientation under normal growth conditions. Therefore, they become polycrystalline when a plurality of crystalline grains are concurrently grown.
FIGS. 1A-1G are diagrams that illustrate a process of amorphous silicon crystallization according to conventional art, which is typically performed on the amorphous silicon on a substrate by an overlap scanning method using a laser at room temperature of between 300.degree. C. and 400.degree. C. Since lateral crystal growth is achieved in the 1 to 2 .mu.m range at these temperatures, the laser beam should scan the substrate in 1 .mu.m steps.
Referring to FIG. 1A, a silicon oxide (SiO.sub.2) layer 101, functioning as a buffer layer, is formed on a glass substrate 100, and an amorphous silicon layer 10, to be crystallized, is formed on the silicon oxide layer 101. A laser beam 50, approximately 1 mm wide, is positioned over the amorphous silicon layer 10. The laser equipment is set to generate a laser beam that has a appropriate energy density and a repeat rate for the process, and that is focused on the amorphous silicon layer 10. The melted portion of the amorphous silicon layer 10 irradiated by the laser beam is referred to as "molten region 10a."
Referring to FIG. 1B, the next step requires moving the emitting position of the laser beam 50 about 1 .mu.m from the previous position. Before the laser beam is emitted toward the substrate from the shifted position, the molten region 10a cools and solidifies, producing a crystal structure transition. During the cooling and solidification processes, different parts of the molten region 10a produce seed crystals that will grow concurrently in different orientations. Because the laser beam is approximately 1 mm wide, the amorphous silicon can crystallize to form a polycrystalline region 10p of about 1 mm wide.
Referring to FIG. 1C, the laser beam 50 is projected onto the substrate from the shifted position. The laser-irradiated portion of the substrate, the silicon layer 10 which includes a large part of the polycrystalline region 10p and an amorphous silicon region of about 1 .mu.m wide, becomes a new molten region 10a at the shifted position.
Referring to FIG. 1D, the next step requires moving the emitting position of the laser beam 50 about 1 .mu.m from the previous position. As previously described with reference to FIG. 1B, before the laser beam is emitted toward the substrate from the newly set emitting position, a crystal structure transition occurs in the amorphous silicon layer's molten region 10a that is formed in the step of FIG. 1C, and accordingly the molten region 10a is cooled and solidified. During the cooling and solidification processes, the boundary of the remaining polycrystalline region 10p that is located on the lateral wall of the molten region 10a acts as a new seed crystal to give a lateral growth in the molten region 10a, forming a single-crystal region 10m. Single-crystal growth occurs for about 1 to 2 .mu.m in width when the substrate is heated to between 300 and 400.degree. C., as described above. Single-crystal lateral growth on this side of the molten region 10a continues until the other part of the molten region 10a produces several seed crystals and grows into small crystals. This part of the molten region 10a will be referred to as "quasi-polycrystalline region 10s".
Referring to FIG. 1E, the laser beam 50 is projected onto the substrate. The laser-irradiated portion of the silicon layer 10, which includes a portion of single-crystal region 10m, quasi-polycrystalline region 10s, and an amorphous silicon region of about 1 .mu.m wide becomes a new (third) molten region 10a.
Referring to FIG. 1F, the next step requires moving the emitting position of the laser beam 50 about 1 .mu.m from the previous position. As previously described with reference to FIG. 1B, before the laser beam is emitted toward the substrate from the new emitting position, a crystal structure transition occurs in the newly created amorphous silicon layer's molten region 10a, the molten region 10a cools and solidifies. During the cooling and solidification process, the boundary of the single-crystal region 10m that is located on the lateral wall of the molten region 10a acts as a new seed crystal to give a lateral growth in the molten region 10a, expanding the single-crystal region 10m. In addition, the quasi-polycrystalline region 10s is formed in the other part of the molten region 10a.
The amorphous silicon layer 10 is melted, cooled and solidified, in a repeated manner, and the entire substrate is eventually irradiated by a laser as shown in FIG. 1G. Part of the first polycrystalline region 10p acts as a seed crystal that has grown to the single-crystal region 10m. The quasi-polycrystalline region 10s remains at the other edge of the single-crystal region 10m. As a result, a single crystal layer can be grown from the amorphous silicon on a glass substrate.
As described above, a conventional crystallization process is carried out at room temperature or between 300.degree. C. and 400.degree. C., and a lateral growth of crystals might be in the 1 to 2 .mu.m range in width at most for each step of laser irradiation. On a large-sized substrate, this crystallization rate is too slow and decreases the production yield. FIG. 2 illustrates the time required for amorphous silicon crystallization. The time for a large-sized glass substrate is given by the following equation: ##EQU1##
With a laser beam length of 150 mm, a laser repeat rate of 300 Hz, a substrate's width of 300 mm, and 2 required scanning operations, the time for conventional single-crystallization of a glass substrate 300.times.350 mm.sup.2 can be calculated by using the above equation as follows: ##EQU2##
As a result, about 40 minutes is required to crystallize amorphous silicon formed on a glass substrate of 300.times.350 mm.sup.2. This means that 1.5 substrate sheets per hour can be crystallized. This low processing rate is mainly due to a short lateral growth length because the crystallization is performed at room temperature or at a temperature between 300.degree. C. and 400.degree. C. in the conventional art. In order to reduce the time for crystallization, the lateral growth length must be increased.
When the lateral growth length is increased to about 5 .mu.m, the time for processing a substrate will dramatically decrease as follows: ##EQU3##
A higher temperature of the substrate is desirable to increase the lateral growth length; for example, the lateral growth length increases to 5 .mu.m when the substrate is heated to approximately 700.degree. C. to 800.degree. C. However, such high temperature process is not suitable for a glass substrate because the glass substrate is warped and/or softened by the high temperatures.